Process for producing aromatic carboxylic acid

ABSTRACT

A process for continuously producing an aromatic carboxylic acid comprising oxidizing an aromatic compound substituted with alkyl groups with molecular oxygen gas in the liquid phase in a solvent containing a lower aliphatic carboxylic acid in the presence of a catalyst comprising heavy metal compounds and a bromine compound, wherein a mother liquor which is obtained after removal of crystals from a reaction liquid of the liquid phase oxidation and contains heavy metal ions and bromine ion as catalyst components is brought into contact with a chelate resin of an anion exchange type to recover the catalyst components. 
     The catalyst components are efficiently recovered, and auxiliary agents in an amount exceeding the equivalent amount and excessive labor are not necessary.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to process for producing an aromaticcarboxylic acid. More particularly, the present invention relates to aprocess for producing an aromatic carboxylic acid, particularly2,6-naphthalenedicarboxylic acid, by the liquid phase oxidation of anaromatic compound substituted with alkyl groups, wherein catalystcomponents are efficiently recovered from a mother liquor obtained afterseparation of crystals from the reaction liquid of the liquid phaseoxidation.

2. Description of the Related Arts

Aromatic carboxylic acids, particularly aromatic dicarboxylic acids suchas terephthalic acid and 2,6-naphthalenedicarboxylic acid, and estersthereof are useful as raw materials for polyesters which are used asfibers and resins.

Aromatic carboxylic acids are produced by liquid phase oxidation ofaromatic compounds substituted with alkyl groups in the presence of aheavy metal catalyst. For example, in processes proposed in thespecifications of Japanese Patent Publication Showa 34(1959)-2666 andJapanese Patent Publication Showa 56(1981)-3337, an aromaticdicarboxylic acid is obtained by oxidation in a solution containing alower aliphatic carboxylic acid in the presence of a catalyst containinga heavy metal, such as cobalt and manganese, and bromine.

In the above processes, the reaction product is obtained in the form ofa slurry containing solid substances after the oxidation, and thearomatic carboxylic acid is obtained from crystals separated bysolid-liquid separation of the slurry. The mother liquor which isobtained as the filtrate after the solid-liquid separation of the slurrycontains useful catalyst components, such as expensive heavy metals suchas cobalt and manganese and bromine compounds. It is necessary that thecatalyst components be recycled in order to use these componentsefficiently.

The simplest method for the recycling is to reuse the mother liquorwithout any treatment. However, various organic impurities formed asbyproducts in the reaction and inorganic substances formed by elution ofmaterials of apparatuses are present in the mother liquor. Theseimpurities are concentrated during the recycling and occasionally haveserious adverse effects on the oxidation.

Therefore, the direct recycling of the mother liquor is limited, andcatalyst components which are not reused by the recycling of the motherliquor must be recovered. Various processes have heretofore beenproposed to recover catalyst components which are not reused by therecycling of a mother liquor.

For example, water and an alkali metal carbonate are added to a residueobtained after removal of a solvent from a mother liquor, and metalcomponents of a catalyst are precipitated in the form of carbonates,which are then dissolved in acetic acid to recover the metal components(Japanese Patent Application Laid-Open No. Showa 48(1973)-66090). Inanother example, oxalic acid is added to a mother liquor which is notrecycled, and metal components of a catalyst are recovered as oxalates(Japanese Patent Application Laid-Open No. Heisei 2(1990)-203939).

It has been known that metal components can be recovered with anionexchange resins from a mother liquor which is not recycled. For example,cobalt and bromine in the mother liquor is adsorbed with a strong basicanion exchange resin after the ratio of bromine to cobalt has beenadjusted in a specific range, and the adsorbed cobalt and bromine areeluted with acetic acid containing water to recover organic substancescontaining cobalt (Japanese Patent Application Laid-Open No. Showa53(1978)-133574). In another example, a mother liquor is first broughtinto contact with an anion exchange resin in the bromide form so thatheavy metals are adsorbed with the anion exchange resin, and theadsorbed heavy metals are subsequently recovered from the ion exchangeresin by elution. The treated mother liquor is then brought into contactwith a weak basic anion exchange resin so that bromine ion is adsorbedwith the anion exchange resin, and the adsorbed bromine ion issubsequently recovered by elution from the anion exchange resin(Japanese Patent Application Laid-Open No. Showa 53(1978)-104590).

When metal components are recovered as carbonates or oxalates, auxiliaryagents such as alkali metal carbonates and oxalic acid are necessary inamounts exceeding the equivalent amounts, and the processes are noteconomically advantageous. Moreover, recovery of the carbonates andoxalates requires complicated operations such as neutralization,precipitation and separation of metal salts and excessive labor.

The processes using an anion exchange resin are superior to the aboveprocesses in that such agents are not necessary. However, in order toachieve complete adsorption of heavy metals, bromine ion is necessary inan amount by mol about twice or more as much as the amount of the heavymetal at the time of the adsorption. The bromine ion is recoveredsimultaneously when the heavy metals are recovered. Therefore, the ratioof bromine ion and the heavy metal components in the oxidation isnaturally restricted, and the oxidation is conducted in the presence ofa large relative amount of bromine ion.

Although it is necessary that the ratio of bromine ion to heavy metalsbe kept at a specific value or more in the oxidation, bromine ion in anexcessively large amount does not show further contribution to theimprovement of the reaction, and the amount of bromine discharged to theoutside of the system as organic bromine compounds in the off-gasincreases. This causes loss of bromine, and the possibility of corrosionalso increases. Therefore, it is not preferable that the ratio ofbromine ion to heavy metals in a catalyst must be kept at a high valuein the liquid phase oxidation.

In the processes using carbonates and oxalates, the metal components areconcentrated to form solid substances. In contrast, metals adsorbed withan anion exchange resin are recovered by elution with acetic acidcontaining a large amount of water. The degree of concentration of theeluted ions is decided by the concentration during the elution, andoccasionally the degree of concentration of the eluted ions isinsufficient. When the degree of concentration of eluted ions isinsufficient, recycling of the recovered catalyst directly into thereaction system increases the concentration of water in the reactor, andthis adversely affects the reaction. Therefore, an additional treatmentsuch as removal of water before the recycling is necessary, and thisconsumes additional energy.

SUMMARY OF THE INVENTION

The above drawbacks of the conventional technologies are improved by thepresent invention. An object of the present invention is to provide aprocess which enables efficient recovery of catalyst components with ananion exchange resin without using an auxiliary agent in an amountexceeding the equivalent amount and without complicated operations orexcessive labor in a process for continuously producing an aromaticcarboxylic acid by liquid phase oxidation of an aromatic compoundsubstituted with alkyl groups in a lower aliphatic carboxylic acid asthe solvent in the presence of a catalyst comprising heavy metalcompounds and a bromine compound.

As the result of extensive studies by the present inventors to achievethe above object in the production of an aromatic carboxylic acid, itwas found that, when a chelate resin of an anion exchange type is usedas the anion exchange resin, the desirable effect can be exhibitedindependently of the ratio of bromine ion to the heavy metal ions duringadsorption and that the catalyst components are efficiently recoveredand the liquid phase oxidation can be carried out advantageously withoutoperations for the oxidation at a larger ratio of bromine ion to theheavy metal ion or for separation of water. The present invention hasbeen completed on the basis of the above knowledge.

Accordingly, the present invention provides a process for continuouslyproducing an aromatic carboxylic acid comprising oxidizing an aromaticcompound substituted with alkyl groups with molecular oxygen gas in theliquid phase in a solvent containing a lower aliphatic carboxylic acidin the presence of a catalyst comprising heavy metal compounds and abromine compound, wherein a mother liquor which is obtained afterremoval of crystals from a reaction liquid of the liquid phase oxidationand contains heavy metal ions and bromine ion as catalyst components isbrought into contact with a chelate resin of an anion exchange type, andsubsequently, the catalyst components are recovered with an elutionliquid.

It was also found by the present inventors that the catalyst componentscan efficiently be recovered in the following preferred embodiments ofthe present invention.

(1) The mother liquor is brought into contact with the chelate resin ata temperature of 50° to 120° C. and the elution liquid is brought intocontact with the chelate resin at a temperature of 20° to 60° C.

(2) The mother liquor is brought into contact with the chelate resin ofan anion exchange type in an ion exchange column so that the heavy metalions and the bromine ion of the catalyst components are adsorbed withthe chelate resin, and subsequently, the elution liquid is passedthrough the ion exchange column in the direction opposite to thedirection of the mother liquor.

(3) After the mother liquor is brought into contact with the chelateresin of an anion exchange type so that the heavy metal ions and thebromine ion are adsorbed with the chelate resin, the liquid remaining ina layer of the chelate resin is replaced with an inert gas.

Particularly when 2,6-naphthalenedicarboxylic acid is produced, anauxiliary agent such as an alkali metal carbonate and oxalic acid isused in a large amount in order to recover the metal components ascarbonates or oxalates because the concentration of heavy metal ions inthe mother liquor of the oxidation is relatively high. Therefore, ananion exchange resin can be used effectively, and the catalystcomponents can be efficiently recovered by the combination of (1) and(2).

Accordingly, the present invention also provides a process forcontinuously producing 2,6-naphthalenedicarboxylic acid comprisingoxidizing a 2,6-dialkylnaphthalene with molecular oxygen gas in theliquid phase in a solvent containing a lower aliphatic carboxylic acidin the presence of a catalyst comprising heavy metal compounds and abromine compound, wherein a mother liquor which is obtained afterremoval of crystals from a reaction liquid of the liquid phase oxidationand contains heavy metal ions and bromine ion as catalyst components isbrought into contact with an anion exchange resin at 50° to 120° C., andsubsequently, the catalyst components are recovered by passing anelution liquid through the chelate resin in a direction opposite to thedirection of the mother liquor at 20° to 60° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic flow chart exhibiting an example of the processfor producing an aromatic carboxylic acid of the present invention. Inthis flow chart, a mother liquor of the oxidation is passed through anion exchange column downward from an upper part of the column, and anelution solution is passed through the ion exchange column upward from alower part of the column.

Numbers in FIG. 1 have the following meanings:

1: A reactor of the liquid phase oxidation

2: A solid-liquid separator

3: An ion exchange column

4: An intermediate tank

5: An elution liquid tank

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of the aromatic compound substituted with alkyl groups which isused as the raw material of the oxidation in the present inventioninclude toluene, p-xylene, m-xylene, pseudocumene,2,6-dimethylnaphthalene, 2,6-diethylnaphthalene and2,6-diisopropylnaphthalene. From these aromatic compounds, aromaticcarboxylic acids such as benzoic acid, terephthalic acid, isophthalicacid, trimellitic acid and 2,6-naphthalenedicarboxylic acid areproduced. The process of the present invention is particularlyadvantageously used for producing 2,6-naphthalenedicarboxylic acid bythe reason described above.

Examples of the lower aliphatic carboxylic acid used as the solvent ofthe oxidation in the present invention include formic acid, acetic acid,propionic acid, butyric acid and mixtures of these acids. Acetic acid ispreferable among these acids from the standpoint of heat stability. Thesolvent may contain water. The content of water is preferably 30% byweight or less. The amount by weight of the solvent is 1 to 20 times,preferably 2 to 10 times, as large as the amount by weight of thearomatic compound substituted with alkyl groups which is the rawmaterial of the oxidation. When the amount of the solvent is less thanthe above range, handling of the slurry of the reaction productoccasionally becomes difficult. When the amount of the solvent exceedsthe above range, the burned amount per the amount of the productoccasionally increases to cause economic disadvantage.

In the present invention, heavy metal compounds and a bromine compoundare used for the oxidation catalyst. The heavy metal compounds compriseat least one compound selected from the group consisting of cobaltcompounds and manganese compounds. Compounds of nickel, cerium orzirconium may be added, where necessary. Examples of the compounds ofcobalt, manganese and other heavy metals include salts of organic acids,hydroxides, halides and carbonates of these metals. Acetates andbromides are preferably used. As the bromine compound, any compoundcontaining bromine can be used as long as the compound is soluble in thereaction system and generates bromine ion. Examples of the brominecompound include inorganic bromine compounds such as hydrogen bromide,sodium bromide and cobalt bromide and organic bromine compounds such asbromoacetic acid and tetrabromoethane. Hydrogen bromide, cobalt bromideand manganese bromide are preferably used.

As for the amount of the catalyst containing the heavy metals, thecobalt compound and/or the manganese compound is added to a solvent insuch an amount that the total concentration of the metal atoms is in therange of 0.01 to 1.5% by weight. When the amount of the catalystcontaining the heavy metals is less than the above range, occasionally,a sufficient activity is not obtained. When the amount of the catalystexceeds the above range, the amount of the catalyst removed togetherwith the crystals of the formed aromatic dicarboxylic acid increases,and the process becomes industrially disadvantageous because of anincrease in cost of the catalyst.

The bromine compound is added in such an amount that the ratio by atomof bromine to the total amount of the metals in the catalyst is 0.1 to10, preferably 0.5 to 5. When the amount of the bromine compound is lessthan the above range, occasionally, a sufficient activity is notobtained. When the amount of the bromine compound exceeds the aboverange, the amount of bromine discharged in the off-gas in the form oforganic bromine compounds increases to increase loss of bromine, and thepossibility of corrosion also increases. Therefore, such amounts are notpreferable.

In the present invention, the aromatic compound substituted with alkylgroups is oxidized with molecular oxygen. Oxygen gas or a gas containingoxygen and an inert gas such as nitrogen and argon is used for thereaction. The air is generally used.

The reactor used for the oxidation may be a reactor equipped with astirrer or a gas atomizer. A reactor equipped with a stirrer ispreferably used because sufficient mixing of the content can beachieved.

The reaction is conducted in a continuous production apparatus. Thereaction may be completed in a single step. However, it is preferablethat a plurality of reactors connected in series are used to increasethe yield of the reaction.

The temperature of the liquid phase oxidation in the present inventionis in the range of 170° to 250° C., preferably 180° to 240° C. When thetemperature is lower than the above range, large amounts of intermediateproducts of the reaction remain in the product. When the temperatureexceeds the above range, loss of the solvent by burning increases.

The pressure of the reactor is not particularly limited as long as thereaction system is kept in the liquid phase at the temperature of thereaction. The pressure is generally 5 to 40 kg/cm², preferably 10 to 30kg/cm².

In the oxidation, the gas containing oxygen is continuously supplied tothe reactor, and the gas which has been used for the reaction iscontinuously discharged to the outside of the reactor as an off-gas. Areflux condenser is attached to the reactor to condense a large amountof the solvent accompanied with the off-gas and water formed by theoxidation. The condensed solvent and water are generally returned to thereactor. However, a portion of the condensed liquid may occasionally betaken out of the reaction system to adjust the concentration of water inthe reactor.

In the process of the present invention, the amount of the gascontaining oxygen which is supplied to the reactor is adjusted so thatthe concentration of oxygen gas is in the range of 0.5 to 5% by volumein the off-gas discharged from the reactor as a dry gas from which waterand the solvent have been removed by condensation. When theconcentration of oxygen in the off-gas is lower than the above range,the amounts of intermediate products increase and, moreover, undesirablephenomena such as coloring of the formed aromatic dicarboxylic acid mayoccasionally take place. When the concentration of oxygen exceeds theabove range, loss of the lower aliphatic carboxylic acid used as thesolvent by oxidation increases, and the cost of an air compressorincreases unnecessarily.

It is preferable that the reaction liquid containing crystals of thearomatic carboxylic acid formed in the oxidation reactor is transferredto another oxidation reactor connected to the first reactor in series,and the oxidation is completed with the gas containing oxygen in thesecond reactor. Where necessary, the resultant reaction liquid istransferred to one or more crystallization tanks connected in series.The obtained product is cooled after the pressure was released and thentransferred to the next solid-liquid separation step.

In the solid-liquid separation step, the slurry formed by the oxidationand containing the aromatic carboxylic acid is separated into crystalsand a mother liquor by a solid-liquid separator. The separation of thecrystals is generally carried out under an atmospheric pressure. Thetemperature of the separation is not particularly limited. Theseparation is carried out generally at a temperature lower than theboiling point of the solvent under an atmospheric pressure, for examplein the range of about 50° to 110° C.

Examples of the separator include a centrifugal sedimentation separator,a centrifugal filter and a vacuum filter.

In the process of the present invention, the mother liquor of theoxidation containing heavy metals such as cobalt, manganese and bromine,which is obtained after separation of crystals from the reaction liquidof the liquid phase oxidation as described above, is brought intocontact with a resin of an ion exchange type by passing the motherliquor through an ion exchange column, and subsequently the catalystcomponents are recovered with an elution liquid. After the mother liquorof the oxidation is brought into contact with the resin, a portion ofthe mother liquor may be returned directly to the reactor of theoxidation. Where necessary, a portion or the entire amount of the motherliquor may further be treated with an anion exchange resin. The crystalsseparated by the solid-liquid separator may be washed with water oracetic acid containing water. The liquid produced by the washing may beadded to the mother liquor of the oxidation, and the resultant mixturemay be passed through a column of an ion exchange resin.

The mother liquor of the oxidation which is treated by the ion exchangecolumn in the present invention (occasionally, referred to hereinafteras a liquid for treatment) is a mixture containing the aromaticcarboxylic acid, the solvent, the catalyst, various organic compounds,such as the unreacted raw material and intermediate products andbyproducts of the reaction, and water formed by the reaction in aconsiderable amount. The content of fine crystals in the liquid fortreatment is 1.0% by weight or less, preferably 0.1% by weight or less.The solid-liquid separation is conducted so as to bring the content offine crystals within this range.

In the process of the present invention, it is particularly preferablethat a chelate resin of an anion exchange type is used as the anionexchange resin. The chelate resin is an ion exchange resin exhibiting aparticularly high selectivity to specific metal ions by forming thechelate bonding. Examples of the chelate resin of an anion exchange typeinclude chelate resins of the picolylamine type, the polyamine type, andthe pyridine type. Any of these resins can be used. Specific examples ofthe chelate resin include UNISELEC UR-3300S (a trade name; manufacturedby UNITICA Co., Ltd.), DIAION CR20 (a trade name; manufactured byMITSUBISHI CHEMICAL Co., Ltd.), SUMICHELATE CR-2 (a trade name;manufactured by SUMITOMO CHEMICAL Co., Ltd.) and DOWEX XFS 4195 (a tradename; manufactured by DOW CHEMICAL Company).

It is the characteristic of the present invention that, among variousanion exchange resins, a chelate resin of an ion exchange type is used.

When a heavy metal is adsorbed with an ordinary anion exchange resin,the metal atom has several bromine atoms coordinated with the metalatom. When the number of the bromine atom coordinated with the metalatom is smaller, the metal atom is not adsorbed because of insufficientstability. Therefore, it is generally necessary that the ratio ofbromine ion to the metal atom of the catalyst (referred to hereinafteras a bromine ratio) in the liquid for the treatment be two or more.

In contrast, the restriction by the bromine ratio is not necessary whenthe chelate resin of an ion exchange type, which is a characteristiccomponent in the present invention, is used. The reasons are consideredto be that (1) a ligand other than bromine such as acetate ion andhydroxy ion can be selected when the chelate resin of an ion exchangetype is used, and that (2) the metal ion adsorbed with the chelate resinhas, as the ligands, two or more bromine atoms which are present in thechelate resin by the ion exchange. The chelate resins have acharacteristic that an adsorbed ion forms a cyclic structure with amultidentate ligand having two or more coordinating atoms, i.e., achelate compound, unlike ordinary ion exchange resins in which an ion isadsorbed at a single coordinating site. Therefore, the latter phenomenon(2) is considered to be more likely to be the reason.

By passing the mother liquor of the oxidation through the ion exchangeresin, cobalt, manganese and bromine of the catalyst components areselectively adsorbed. Because other metal impurities are only negligiblyadsorbed by the above ion exchange resin, cobalt, manganese and bromineof the catalyst components can be simultaneously recovered by passing anelution liquid through the resin and can be recycled as the catalyst forthe oxidation.

The ion exchange resin may be used for a liquid for the treatmentwithout any pretreatment. However, it is preferable that the resin isbrought into contact with a solution, such as an acetic acid solution,containing bromine and converted into the bromide form in advance.

To convert the ion exchange resin into the bromide form, the resin iswashed with water or a solution of acetic acid which contains bromineion, for example, as sodium bromide or hydrobromic acid, and,thereafter, excess bromine is removed by washing with acetic acid oracetic acid containing 15% by weight or more of water. It is preferablethat acetic acid used for the washing has a smaller concentration ofwater than that of the mother liquor of the reaction which is the liquidfor treatment.

In the present invention, the concentration of water in the liquid fortreatment passed through the ion exchange column is 0 to 15% by weight,preferably 5 to 12% by weight. In general, the mother liquor of theoxidation contains 5 to 10% by weight of water, and the adjustment ofthe concentration of water is not necessary before the mother liquor issupplied to the ion exchange column in many cases. When a liquidproduced by washing the crystals is added to the mother liquor asdescribed above and the concentration of water in the mother liquorexceeds the above range, the concentration of water is adjusted into theabove range by distillation or the like method.

The elution liquid which is used for elution and recovery of the metalsadsorbed with the ion exchange resin is preferably water or an aliphaticcarboxylic acid containing water. The same solvent as that used for theoxidation is used as the elution liquid. Acetic acid is generally used.The concentration of water in acetic acid is 15% by weight or more,preferably 25% by weight or more. The condensed liquid separated in thereflux cooler of the reactor of the oxidation may be used as the elutionliquid.

The temperature of the liquid for treatment is in the range of 50° C. to120° C., preferably in the range of 70° to 110° C., when the liquid isbrought into contact with the anion exchange resin. A temperatureexceeding the above range is not preferable because a higher temperaturehas restriction due to heat resistance of the anion exchange resin andthe boiling point of the solvent (acetic acid). When the temperature islower than this range, the activity of adsorption of the anion exchangeresin occasionally decreases.

The temperature of the elution solution during recovery of the catalystcomponents is selected in the range of 20° to 60° C., preferably 30° to50° C. When the temperature is higher than this range, the efficiency ofthe elution may decrease because the activity of adsorption of the resinis higher. When the temperature is lower than this range, the cost ofcooling increases. Therefore, such temperatures are not preferable.

In the present invention, it is preferable that the temperature of themother liquor which is in contact with the anion exchange resin is keptat a relatively high temperature, i.e., a temperature in the range of50° to 120° C., preferably in the range of 70° to 110° C., and thetemperature of the elution liquid used for recovery of the catalystcomponents is kept at a relatively low temperature, i.e., a temperaturein the range of 20° to 60° C., preferably in the range of 30° to 50° C.,to achieve more efficient recovery of the catalyst components. This isone of the characteristics of the present invention found by the presentinventors.

The above characteristic is based on the fact that the equilibriumbetween the concentration of the metal ion adsorbed with the anionexchange resin and the concentration of the metal ion in the solutionchanges with the temperature to a great degree. In other words, at ahigh temperature of the environment, the activity of adsorption of theanion exchange resin is high and the condition is relativelyadvantageous for adsorption; on the other hand, at a low temperature ofthe environment, the activity decreases and the condition is relativelyadvantageous for elution; and the above characteristic is based on thisfact.

A system using an anion exchange resin is essentially a system in whichadsorption and desorption are repeated utilizing the difference instability of a substance for adsorption which is obtained by changingthe concentration of water in the solvent. The swing of the temperaturesuch as that described above can effectively be applied for chelateresins.

The system in which the temperature is changed between adsorption anddesorption is well known for conventional adsorbents as the temperatureswing system for repeated absorption and desorption. The pressure swingsystem is also well known. However, such systems are not generally knownfor a process using an ion exchange resin.

It is effective for more fully exhibiting the function of the system ofthe present invention that the heavy metal ions and bromine ion of thecatalyst components are adsorbed with the anion exchange resin bypassing the mother liquor through the ion exchange resin, and then thecatalyst components are recovered by passing the elution solutionthrough the ion exchange resin in a direction opposite to the directionof the mother liquor of the oxidation. Particularly, it is preferablethat the mother liquor of the oxidation is passed through the ionexchange resin downward from the upper part of the ion exchange column,and the elution liquid is passed through the column upward from thelower part of the column.

It is also one of the characteristics of the present invention found bythe present inventors that the direction of flow of the mother liquor ofthe oxidation is made opposite to the direction of flow of the elutionliquid.

In the process for recovery of the catalyst components using an anionexchange resin reported in Japanese Patent Application Laid-Open No.Showa 53(1978)-104590, which is described above as the conventionalprocess, a mother liquor of the liquid phase oxidation is supplied atthe top of an ion exchange column so that the catalyst components areadsorbed, and then an elution liquid is supplied at the top of the ionexchange column to recover the metals of the catalyst components. Inaccordance with this process, there is the tendency that the catalystcomponents adsorbed in low concentrations are first eluted andrecovered, and the concentrations of the recovered catalyst componentsgradually increase.

In contrast, in the present invention, the catalyst components adsorbedin high concentrations are first eluted and recovered because theelution liquid is passed through the column in the direction opposite tothe mother liquor of the oxidation. When the yield of recovery in thesame period of time are compared between these processes, it is apparentthat the process of the present invention can achieve a higher yield ofrecovery of the catalyst components.

In the present invention, it is preferable that the mother liquor of theoxidation is passed through the ion exchange column downward from theupper part of the column, and the elution solution is passed through thecolumn upward from the lower part of the column. When the directions ofthe liquids are selected in this manner, the catalyst components whichare adsorbed in high concentrations at upper parts of the ion exchangecolumn can be eluted first, and the elution and recovery of the catalystcomponents can be carried out more efficiently than the case in whichthe elution liquid is passed through the column downward from the upperpart of the column.

Moreover, when the mother liquor of the oxidation is passed through theion exchange column downward from the upper part of the column, and theelution solution is passed through the column upward from the lower partof the column as described above, crystals of the aromatic carboxylicacid and organic impurities which have leaked through the solid-liquidseparation or are precipitated because of decrease in the temperature ofthe mother liquor are discharged from the upper part of the ion exchangecolumn during the elution. Thus, accumulation of such crystals andimpurities can be prevented.

Therefore, problems, such as troubles in operation accompanied withincrease in the pressure difference caused by the accumulation describedabove, decrease in the ability of exchange caused by adhesion of thecrystals and organic impurities described above to the surface of theion exchange resin and decrease in the adsorbing ability caused byuneven flow in the inner parts of the resin due to the migratedcrystals, can be prevented by adopting the above method of passing theliquids. Passing the eluting liquid through the column upward from thelower part of the column has a further advantages that the ion exchangeresin which has been pressed into a tight condition can be relaxed to aloose condition.

When the elution liquid is passed through the column upward from thelower part of the column, the operation is made at a low space velocity(SV) or a structure working as a stopper is formed at an upper part ofthe resin layer in order to prevent pulverization of the resin.

It is effective for removing organic impurities attached to the resinthat acetic acid containing water in the same concentration as that ofthe mother liquor or lower is passed through the column from the upperpart or the lower part of the column between the operations of theadsorption and the elution. The yield of recovery of the catalystcomponents can also be increased.

The present invention will be described more specifically with referenceto a figure showing a schematic flow chart of the process in thefollowing. FIG. 1 shows an example of the process in which the motherliquor of the oxidation is passed through the ion exchange columndownward from the upper part of the column, and the elution liquid ispassed through the column upward from the lower part of the column. InFIG. 1, a mixture of raw materials 7 is transferred to a reactor of theliquid phase oxidation 1 to carry out the oxidation. The reaction liquidobtained by the liquid phase oxidation is a slurry containing thearomatic carboxylic acid of the reaction product. The reaction liquid isseparated into a solid and a liquid in a solid-liquid separator 2, and acake of crystals of crude aromatic dicarboxylic acid 8 is obtained.

In the adsorption step, a mother liquor of the reaction obtained by theseparation is supplied at the upper part 9 of an ion exchange column 3.In the ion exchange column 3, the supplied mother liquor is sufficientlyheat insulated so that the temperature of the elution liquid is keptunchanged by the outside temperature.

In the ion exchange column 3, ions of cobalt, manganese, and brominecontained in the mother liquor are separated by adsorptionsimultaneously. Organic compounds such as organic impurities and metalimpurities other than cobalt and manganese are not adsorbed and purgedthrough the passage 10 at the lower part of the column together with thereaction liquid of the oxidation.

In the elution step, cobalt, manganese and bromine adsorbed in the ionexchange column are simultaneously eluted by passing an elution liquid6, such as an aqueous solution of acetic acid, through the column upwardfrom the lower part of the column. The elution solution used here issupplied from an elution liquid tank 5. The aqueous solution of aceticacid which flows out of the top of the column and contains elutedcobalt, manganese and bromine is collected into an intermediate tank 4and recycled to utilize the catalyst again for the liquid phaseoxidation.

In the present invention, it is preferable that the liquid remaining inthe ion exchange layer after the adsorption has been completed isreplaced with an inert gas. Nitrogen or carbon dioxide can be used asthe inert gas. A gas discharged from the oxidation step can also be usedas the inert gas. The ion exchange resin in which the remaining liquidhas been replaced with the inert gas is used in the elution step. It ispreferable that the elution liquid is supplied upward from the lowerpart of the column. When a liquid is supplied into a column containingpores downward from the upper part of the column, there is thepossibility that bubbles are contained in the liquid. When the liquid issupplied upward, this possibility can be eliminated.

It is also one of the characteristics of the present invention found bythe present inventors that the liquid remaining in the column of the ionexchange resin is replaced with an inert gas after the adsorption of themetal ions and bromine ion with the chelate resin of an anion exchangetype have been completed.

The liquid remaining in the resin layer immediately after the adsorptionhas been completed is the mother liquor from which the metal ions andbromine ion have been removed by the adsorption, and this mother liquornaturally contains metals and water in low concentrations. When thecolumn is switched to elution immediately after the adsorption iscompleted, the liquid remaining in the resin layer is mixed with theelution liquid, and the elution liquid is diluted. Moreover, it isdelayed that the concentration of water in the liquid in the inner partof the resin layer reaches the desirable value for the elution. Thesephenomena are not preferable for concentration of the catalystcomponents.

To solve the above problems, the elution liquid may be used after asuitable interval of time from the end of the adsorption step. However,the concentration of the catalyst components in the elution liquid isparticularly high in the initial period of the elution. Moreover, thereverse mixing of the elution liquid and the mother liquid cannot beprevented completely. Naturally, the mother liquor can be purged only toa limited extent.

Therefore, when the mother liquor remaining in the resin layer isdischarged to the outside of the system with an inert gas and theelation is started thereafter, the mother liquor remained in the resinlayer is completely replaced with the inert gas, and an elution liquidhaving a high concentrations of metal ions and bromine ion can beobtained.

To summarize the advantage obtained by the present invention, cobalt andmanganese which are valuable substances used in the liquid phaseoxidation of an aromatic compound substituted with alkyl groups can beefficiently recovered and recycled by using a chelate resin of an anionexchange type.

The present invention has the following characteristics as the processfor recovering the metal components of the catalyst and enables theindustrially advantageous recovery of metal components of the catalyst.

(1) An alkali metal carbonate or oxalic acid which is required inprocesses for recovery of the metal components in the form of carbonatesor oxalates is not necessary, and complicated operations such asneutralization, precipitation and separation are not necessary either.

(2) An exceedingly high ratio of bromine to metals is not necessary.Therefore, loss of bromine compounds can be reduced, and the possibilityof corrosion decreases.

(3) A highly concentrated liquid for recycling can be obtained, and themetal components of the catalyst can be recovered efficiently by settingdifferent temperatures for the mother liquor of the oxidation at thetime of adsorption of the catalyst components and for the elation liquidat the time of recovery of the catalyst components, by passing themother liquor of the oxidation and the elution liquid through the anionexchange resin in the directions opposite to each other, and by purgingthe mother liquor remaining in the resin layer to the outside of thesystem before the elution.

EXAMPLES

The present invention is described more specifically with reference toexamples. However, the present invention is not limited to the examples.

In the following Examples and Comparative Examples, an ion exchangecolumn of double glass tubes was packed with 150 ml of an anion exchangeresin and heat insulated by circulation of water of the same temperatureas that of a liquid supplied into the column so that the temperature ofthe supplied liquid is not affected by the outside temperature.

In Examples 1 to 6, the ion exchange resin was pretreated by passing 200ml of a solution of acetic acid containing 10% by weight of hydrobromicacid through the resin to convert the resin into the bromide form,followed by passing a solution of acetic acid containing 20% by weightof water through the resin to remove excess hydrobromic acid. A solutionof acetic acid containing 50% by weight of water was used as the elationliquid.

EXAMPLE 1

A reaction product obtained by the liquid phase oxidation of pxylene inacetic acid was cooled to 80° C., and a mother liquor of the oxidationwas obtained after the solid-liquid separation of the resultant productusing a vacuum filter. The mother liquor was used as the raw materialliquid in the recovery process. This raw material liquid contained 700ppm of cobalt and 400 ppm of manganese and had a ratiobromine/(cobalt+manganese) of 0.67.

An ion exchange column was packed with 150 ml of a chelate resin of ananion exchange type (manufactured by SUMITOMO CHEMICAL Co., Ltd.; CR-2)which had been pretreated as described above, and hot water adjusted to80° C. was circulated through a jacket of the column. The raw materialliquid in an amount of 1,500 g was introduced into the ion exchangecolumn downward from the upper part of the column. The liquid fortreatment which had passed through the column was discharged into awaste liquid tank. Then, water adjusted to 30° C. was circulated throughthe jacket of the column, and the elution liquid adjusted to 30° C. wassupplied upward from the lower part of the ion exchange column until theconcentration of cobalt at the outlet of the ion exchange column reached10 ppm or less.

The yield of recovery of cobalt was 99.5%, and the yield of recovery ofmanganese was 80%. The amount of the liquid required for the elution was450 g.

EXAMPLE 2

The same procedures as those conducted in Example 1 were conductedexcept that the temperature of the jacket during the elution was kept at60° C. The amount of the liquid required for the elution to obtain thesame yields of recovery as those in Example 1 was 500 g.

EXAMPLE 3

The same procedures as those conducted in Example 1 were conductedexcept that the temperature of the jacket during the elution was kept at80° C. The amount of the liquid required for the elution to obtain thesame yields of recovery as those in Example 1 was 750 g.

EXAMPLE 4

The same procedures as those conducted in Example 1 were conductedexcept that the direction of the flow of the mother liquor during theadsorption was the same as that of the elution liquid, i.e., upwardthrough the column. The amount of the liquid required for the elution toobtain the same yields of recovery as those in Example 1 was 910 g.

EXAMPLE 5

The same procedures as those conducted in Example 1 were conductedexcept that the liquid inside the column was purged to the outside ofthe system with nitrogen after the adsorption, and then the elutionliquid was supplied upward. The amount of the liquid required for theelution to obtain the same yields of recovery as those in Example 1decreased to 400 g.

EXAMPLE 6

A reaction product obtained by the liquid phase oxidation of2,6-dimethylnaphthalene in acetic acid was cooled to 80° C., and amother liquor of the oxidation was obtained after the solid-liquidseparation of the resultant product using a decanter type centrifuge.The mother liquor was kept at 80° C. and used as the raw material liquidin the recovery process. This raw material liquid contained 1,600 ppm ofcobalt and 1,000 ppm of manganese and had a ratiobromine/(cobalt+manganese) of 0.83.

An ion exchange column was packed with 150 ml of a chelate resin of ananion exchange type (manufactured by SUMITOMO CHEMICAL Co., Ltd.; CR-2)which had been pretreated as described above, and hot water adjusted to80° C. was circulated through a jacket of the column. The raw materialliquid in an amount of 600 g was introduced into the ion exchange columndownward from the upper part of the column. The liquid for treatmentwhich had passed through the column was discharged into a waste liquidtank. Then after the liquid remaining in the ion exchange column afterthe adsorption was removed with nitrogen gas to the outside of thesystem, water adjusted to 30° C. was circulated through the jacket ofthe column, and the elution liquid adjusted to 30° C. was suppliedupward from the lower part of the ion exchange column until theconcentration of cobalt at the outlet of the ion exchange column reached10 ppm or less.

The yield of recovery of cobalt was 99.3%, and the yield of recovery ofmanganese was 80%. The amount of the liquid required for the elution was400 g.

EXAMPLE 7

A reaction product obtained by the liquid phase oxidation of2,6-dimethylnaphthalene in acetic acid was cooled to 80° C., and amother liquor of the oxidation was obtained after the solid-liquidseparation of the resultant product using a decanter type centrifuge.Hydrobromic acid was added to the mother liquor to prepare a liquid fortreatment supplied to the ion exchange columnbromine/(cobalt+manganese)=1.2!. The composition of this liquid is shownin Table 1. The liquid for treatment was kept at 80° C. and used in thenext catalyst recovery step.

An ion exchange column was packed with 150 ml of a resin of a strongbasic ion exchange type (manufactured by ORGANO Co., Ltd.; IRA-900)which had been pretreated as described above, and hot water adjusted to80° C. was circulated through a jacket of the column. The adsorptionstep for 270 minutes and the elution step for 90 minutes weresuccessively repeated as follows:

In the adsorption step, the liquid for treatment was introduced into thecolumn downward from the upper part of the column at the speed of 400ml/hour for 270 minutes. The liquid for treatment which had passedthrough the ion exchange column was discharged into a waste liquid tank.

In the elution step, an elution liquid, which was acetic acid containingabout 30% by weight of a condensate separated from the reflux cooler ofthe reactor of the oxidation, was introduced into the ion exchangecolumn upward from the lower part of the column at the speed of 400ml/hour for 90 minutes. The elution liquid which had passed through theion exchange column was discharged into a recovery liquid tank.

The above operations were carried out continuously for 90 days. Theoperations in the above steps could be conducted without any problem.The yields of recovery of the components after 3 days and 90 days of thetest are shown in Table

                  TABLE 1    ______________________________________            composition of liquid                            yield of recovery            for treatment   after 3 days                                      after 90 days            (ppm)           (%)       (%)    ______________________________________    cobalt  680             97        95    manganese            2380            51        45    bromine 5250            95        94    ______________________________________

EXAMPLE 8

The same reaction product as that used in Example 7, i.e., the reactionproduct obtained by the liquid phase oxidation of2,6-dimethylnaphthalene in acetic acid, was cooled to 100° C., and amother liquor of the oxidation was obtained after the solid-liquidseparation of the resultant product using a decanter type centrifuge.Hydrobromic acid was added to the mother liquor to prepare a liquid fortreatment. The liquid for treatment was kept at 100° C. and used in thenext catalyst recovery step.

An ion exchange column was packed with 150 ml of a resin of a weak basicion exchange type (manufactured by ORGANO Co., Ltd.; IRA-96SB) which hadbeen pretreated as described in Example 7, and hot water adjusted to 97°C. was circulated through a jacket of the column. The adsorption stepfor 270 minutes and the elution step for 90 minutes were successivelyrepeated as follows:

In the adsorption step, the liquid for treatment was introduced into thecolumn downward from the upper part of the column at the speed of 400ml/hour for 270 minutes. The liquid for treatment which had passedthrough the ion exchange column was discharged into a waste liquid tank.

In the elution step, the same elution liquid as that used in Example 7was introduced into the ion exchange column upward from the lower partof the column at the speed of 400 ml/hour for 90 minutes. The elutionliquid which had passed through the ion exchange column was dischargedinto a recovery liquid tank.

The above operations were carried out continuously for 90 days. Theoperations in the above steps could be conducted without any problem.The yields of recovery of the components after 3 days and 90 days of thetest are shown in Table

                  TABLE 2    ______________________________________                   yield of recovery                   after 3 days                             after 90 days                   (%)       (%)    ______________________________________    cobalt         99        98    manganese      54        51    bromine        96        93    ______________________________________

COMPARATIVE EXAMPLE 1

The same procedures as those conducted in Example 7 were conductedexcept that, in the elution step, the elution liquid was introduced intothe ion exchange column downward from the upper part of the columnsimilarly to the adsorption step. In other words, the direction of flowof the liquid was the same in the adsorption step and the elution stepin Examples 7 and 8, while the direction of flow of the liquid wasopposite in the adsorption step and the elution step in Examples 7 and8.

After the operation was continued for some time, fine crystals containedin the liquid for treatment were accumulated on the ion exchange resin.After 2 days of the operation, difference in the pressure at the twosides of the resin layer increased, and the operation could not becontinued.

COMPARATIVE EXAMPLE 2

The same procedures as those conducted in Example 7 were conductedexcept that the temperature of water circulated through the jacket wasadjusted to 25° C., and the operation was continued for 3 days. Theresult is shown in Table

                  TABLE 3    ______________________________________                yield of recovery after 3 days                (%)    ______________________________________    cobalt      55    manganese   13    bromine     37    ______________________________________

What is claimed is:
 1. A process for continuously producing an aromaticcarboxylic acid comprising oxidizing an aromatic compound substitutedwith alkyl groups with molecular oxygen gas in the liquid phase in asolvent containing a lower aliphatic carboxylic acid in the presence ofa catalyst comprising heavy metal compounds and a bromine compound,wherein a mother liquor which is obtained after removal of crystals froma reaction liquid of the liquid phase oxidation and contains heavy metalions and bromine ion as catalyst components is brought into contact witha chelate resin of an anion exchange type, and subsequently, thecatalyst components are recovered with an elution liquid.
 2. A processaccording to claim 1, wherein the mother liquor is brought into contactwith the chelate resin at a temperature of 50° to 120° C., and theelution liquid is brought into contact with the chelate resin at atemperature of 20° to 60° C.
 3. A process according to claim 1, whereinthe mother liquor is brought into contact with the chelate resin of ananion exchange type in an ion exchange column so that the heavy metalions and the bromine ion of the catalyst components are adsorbed withthe chelate resin, and subsequently, the elution liquid is passedthrough the ion exchange column in the direction opposite to thedirection of the mother liquor so that the catalyst components arerecovered.
 4. A process according to claim 3, wherein the mother liquoris passed through the ion exchange column downward from an upper partthereof so that the heavy metal components and the bromine ion of thecatalyst components are adsorbed with the chelate resin, andsubsequently the elution liquid is passed through the ion exchangecolumn upward from a lower part thereof so that the catalyst componentsare recovered.
 5. A process according to claim 1, wherein, after themother liquor is brought into contact with the chelate resin of an anionexchange type so that the heavy metal ions and the bromine ion areadsorbed with the chelate resin, the liquid remaining in a layer of thechelate resin is replaced with an inert gas.
 6. A process according toclaim 1, wherein the elution liquid is a solution of acetic acidcontaining 15% by weight or more of water.
 7. A process according toclaim 1, wherein a condensate separated in a reflux cooler of anoxidation reactor for the liquid phase oxidation is used as the elutionliquid.
 8. A process according to claim 1, wherein the aromatic compoundsubstituted with alkyl groups is 2,6-dimethylnaphthalene, and thearomatic carboxylic acid produced by the process is2,6-naphthalenedicarboxylic acid.
 9. A process for continuouslyproducing 2,6-naphthalenedicarboxylic acid comprising oxidizing a2,6-dialkylnaphthalene with molecular oxygen gas in the liquid phase ina solvent containing a lower aliphatic carboxylic acid in the presenceof a catalyst comprising heavy metal compounds and a bromine compound,wherein a mother liquor which is obtained after removal of crystals froma reaction liquid of the liquid phase oxidation and contains heavy metalions and bromine ion as catalyst components is brought into contact withan anion exchange resin at 50° to 120° C., and subsequently, thecatalyst components are recovered by passing an elution liquid throughthe resin in a direction opposite to the direction of the mother liquorat 20° to 60° C.